Gas turbine engines, such as those used to power modern commercial aircraft, include a compressor for pressurizing a supply of air, a combustor for burning a hydrocarbon fuel in the presence of the pressurized air, and a turbine for extracting energy from the resultant combustion gases. In aircraft engine applications, the compressor, combustor and turbine are disposed about a central engine axis with the compressor disposed axially upstream of the combustor and the turbine disposed axially downstream of the combustor.
An exemplary twin wall combustor features an annular combustion chamber defined between a radially inward liner and radially outward shell extending aft from a forward bulkhead. The radially inward liner forms a heat shield. The radially outward shell extends circumferentially about and is radially spaced from the inward liner. Arrays of circumferentially distributed air admittance holes penetrate the outward shell and the inward liner at multiple axial locations along the length of the combustion chamber. Cooling air passes through the holes in the outer shell and then again through the holes in the inner liner, and finally into the combustion chamber. A plurality of circumferentially distributed fuel injectors and associated air passages are mounted in the forward bulkhead. The fuel injectors project into the forward end of the combustion chamber to supply the fuel. The associated air passages impart a swirl to inlet air entering the forward end of the combustion chamber at the bulkhead to provide rapid mixing of the fuel and inlet air. Commonly assigned U.S. Pat. Nos. 6,606,861; 6,810,673 and 7,094,441; the entire disclosures of which are hereby incorporated herein by reference as if set forth herein, disclose exemplary prior art annular combustors for gas turbine engines.
Combustion of the hydrocarbon fuel in air inevitably produces oxides of nitrogen (NOx). NOx emissions are the subject of increasingly stringent controls by regulatory authorities. One combustion strategy for minimizing NOx emissions from gas turbine engines is referred to as rich burn, quick quench, lean burn (RQL) combustion. The RQL combustion strategy recognizes that the conditions for NOx formation are most favorable at elevated combustion flame temperatures, i.e. when the fuel-air ratio is at or near stoichiometric. A combustor configured for RQL combustion includes three serially arranged combustion zones: a fuel-rich combustion zone at the forward end of the combustor, a quench or dilution zone that involves the conversion of rich combustion to lean combustion, and a lean combustion zone axially aft of the quench or dilution zone. Thus, the combustion process in a combustor configured for RQL combustion has two governing states of combustion: a first state in the forward portion of the combustor that is stoichiometrically fuel-rich and a second state in a downstream portion of the combustor that is stoichiometrically fuel-lean.
During engine operation with RQL combustion, a portion of the pressurized air discharged from the compressor is directed through a diffuser to enter the combustion chamber through the inlet air swirlers to support rich-burn combustion. Concurrently, the fuel injectors introduce a stoichiometrically excessive quantity of fuel into the front portion of the combustor. The resulting stoichiometrically rich fuel-air mixture is ignited and burned to partially release the energy content of the fuel. The fuel rich character of the mixture inhibits NOx formation in the rich burn zone by suppressing the combustion flame temperature. It also resists blowout of the combustion flame during certain operating conditions or any abrupt transients to engine power and promotes good ignition of the combustor.
The fuel rich combustion products generated in the first zone of combustion propagate downstream where the combustion process continues. Pressurized air from the compressor enters the combustion chamber radially through a row of circumferentially spaced dilution air admission holes. The additional air admitted through these dilution air holes mixes with the combustion products from the first zone to support further combustion and release additional energy from the fuel. The air also progressively deriches the fuel rich combustion gases as these gases flow axially through and mix with the air introduced in the quench region. Initially, with the dilution air addition, the fuel-air ratio of the combustion products becomes less fuel rich approaching a stoichiometric composition, causing an attendant rise in the combustion flame temperature. Since the quantity of NOx produced in a given time interval increases exponentially with flame temperature, significant quantities of NOx can be produced during the initial quench process where the combustion is rich. As quenching continues, the fuel-air ratio of the combustion products rapidly convert through the stoichiometric state to become fuel lean, causing an attendant reduction in the flame temperature. However, until the mixture is diluted to a fuel-air ratio substantially lower than stoichiometric, the flame temperature remains high enough to generate appreciable quantities of NOx.
One advantage of a twin wall arrangement is that an assembled twin wall arrangement is structurally stronger. A disadvantage to the twin wall arrangement, however, is that high-temperature zones of localized, near-stoichiometric combustion conditions, commonly called hot spots, can occur despite the fuel-rich nature of the forward portion and the fuel-lean nature of the aft portion of a RQL combustion chamber. Therefore, thermal maldistribution must be accounted for closely. Different zones of the combustor will experience different amounts of heat, resulting in local hot zones and the associated stress and strain. If the thermal combustor design does not account for maldistribution of thermal loads, then the usable life of the combustor may be negatively affected.